Electrostatic bimorph actuator

ABSTRACT

An electrostatic bimorph actuator includes a cantilevered flexible bimorph arm that is secured and insulated at one end to a planar substrate. In an electrostatically activated state the bimorph arm is generally parallel to the planar substrate. In a relaxed state, residual stress in the bimorph arm causes its free end to extend out-of-plane from the planar substrate. The actuator includes a substrate electrode that is secured to and insulated from the substrate and positioned under and in alignment with the bimorph arm. An electrical potential difference applied between the bimorph arm and the substrate electrode imparts electrostatic attraction between the bimorph arm and the substrate electrode to activate the actuator. As an exemplary application in which such actuators could be used, a microelectrical mechanical optical display system is described.

FIELD OF THE INVENTION

The present invention relates to microelectromechanical system (MEMS)actuators and, in particular, to electrostatic microelectricalmechanicalsystem (MEMS) actuators that employ a bimorph construction.

BACKGROUND AND SUMMARY OF THE INVENTION

Microelectromechanical system (MEMS) actuators provide control of verysmall components that are formed on semiconductor substrates byconventional semiconductor (e.g., CMOS) fabrication processes. MEMSsystems and actuators are sometimes referred to as micromachinedsystems-on-a-chip. MEMS systems can be used in a wide range ofapplications.

One of the conventional MEMS actuators is the electrostatic actuator orcomb drive. Commonly, such actuators include two comb structures thateach have multiple comb fingers aligned in a plane parallel to asubstrate. The fingers of the two comb structures are interdigitatedwith each other. Potential differences applied to the comb structuresestablish electrostatic interaction between them, thereby moving thecomb structures toward and away from each other. The conventionalelectrostatic actuator or comb drive is limited to motion generallywithin or parallel to the plane of the underlying substrate.

The present invention includes an electrostatic bimorph actuator thatincludes a cantilevered flexible bimorph arm that is secured at one endto a planar substrate. In an electrostatically activated state thebimorph arm is generally parallel to the planar substrate. In a relaxedstate, residual stress in the bimorph arm causes its free end to extendout-of-plane from the planar substrate.

In one implementation the actuator includes a substrate electrode thatis secured to, but electrically isolated from the substrate andpositioned under and in alignment with the bimorph arm. An electricalpotential difference applied between the bimorph arm and the substrateelectrode imparts electrostatic attraction between the bimorph arm andthe substrate electrode to activate the actuator. As an exemplaryapplication in which such actuators could be used, a microelectricalmechanical optical display system is described.

Additional objects and advantages of the present invention will beapparent from the detailed description of the preferred embodimentthereof, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1–15 are cross-section views of a general multi-user MEMS processknown in the prior art for fabricating microelectrical mechanicaldevices. Cross-hatching is omitted to improve clarity of the prior artstructure and process depicted.

FIG. 16 is a schematic side view of a microelectrical mechanical (MEMS)optical display system illustrating an exemplary operating environmentfor an electrostatic actuator of the present invention.

FIG. 17 is a schematic side view of another implementation of amicroelectrical mechanical (MEMS) optical display system illustrating anexemplary operating environment for an electrostatic actuator of thepresent invention.

FIGS. 18 and 19 are schematic side views of an electrostatic bimorphMEMS actuator of the present invention in respective activated andrelaxed states.

FIG. 20 is a plan view of an electrostatic bimorph MEMS actuator.

FIGS. 21 and 22 are side views of the electrostatic bimorph MEMSactuator of FIG. 20 in respective activated and relaxed states.

FIG. 23 is a schematic diagram of a 2×2 array of electrostatic bimorphMEMS actuators having a storage or memory capability.

FIG. 24 is a schematic diagram of a 50×50 array of electrostatic bimorphMEMS actuators having a storage or memory capability.

FIG. 25 is a flow diagram of a row-sequential addressing method.

FIG. 26 is a graph illustrating hysteresis characteristics of anelectrostatic bimorph MEMS actuator with respect to applied voltagedifferentials.

FIG. 27 is a schematic sectional side view of a mirror portion of oneimplementation of an electrostatic bimorph MEMS actuator having acomposite structure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

To assist with understanding the present invention, the generalprocedure for fabricating micromechanical devices using the MUMPsprocess is explained with reference to FIGS. 1–15.

The MUMPs process provides three-layers of conformal polysilicon thatare etched to create a desired physical structure. The first layer,designated POLY 0, is coupled to a supporting wafer, and the second andthird layers, POLY 1 and POLY 2, respectively, are mechanical layersthat can be separated from underlying structure by the use ofsacrificial layers that separate layers and are removed during theprocess.

The accompanying figures show a general process for building amicro-motor as provided by the MEMS Technology Applications Center, 3021Cornwallis Road, Research Triangle Park, N.C.

The MUMPs process begins with a 100 mm n-type silicon wafer 10. Thewafer surface is heavily doped with phosphorus in a standard diffusionfurnace using POCl 3 as the dopant source. This reduces chargefeed-through to the silicon from electrostatic devices subsequentlymounted on the wafer. Next, a 600 nm low-stress Low Pressure ChemicalVapor Deposition (LPCVD) silicon nitride layer 12 is deposited on thesilicon as an electrical isolation layer. The silicon wafer and siliconnitride layer form a substrate.

Next, a 500 nm LPCVD polysilicon film—POLY 0 14—is deposited onto thesubstrate. The POLY 0 layer 14 is then patterned by photolithography; aprocess that includes coating the POLY 0 layer with a photoresist 16,exposing the photoresist with a mask (not shown) and developing theexposed photoresist to create the desired etch mask for subsequentpattern transfer into the POLY 0 layer (FIG. 2). After patterning thephotoresist, the POLY 0 layer 14 is etched in a Reactive Ion Etch (RIE)system (FIG. 3).

With reference to FIG. 4, a 2.0 μm phosphosilicate glass (PSG)sacrificial layer 18 is deposited by LPCVD onto the POLY 0 layer 14 andexposed portions of the nitride layer 102. This PSG layer, referred toherein as a First Oxide, is removed at the end of the process to freethe first mechanical layer of polysilicon, POLY 1 (described below) fromits underlying structure; namely, POLY 0 and the silicon nitride layers.This sacrificial layer is lithographically patterned with a DIMPLES maskto form dimples 20 in the First Oxide layer by RIE (FIG. 5) at a depthof 750 nm. The wafer is then patterned with a third mask layer, ANCHOR1,and etched (FIG. 6) to provide anchor holes 22 that extend through theFirst Oxide layer to the POLY 0 layer. The ANCHOR 1 holes will be filledin the next step by the POLY 1 layer 24.

After the ANCHOR1 etch, the first structural layer of polysilicon (POLY1) 24 is deposited at a thickness of 2.0 μm. A thin 200 nm PSG layer 26is then deposited over the POLY 1 layer 24 and the wafer is annealed(FIG. 7) to dope the POLY 1 layer with phosphorus from the PSG layers.The anneal also reduces stresses in the POLY 1 layer. The POLY 1 and PSGmasking layers 24, 26 are lithographically patterned to form thestructure of the POLY 1 layer.

After etching the POLY 1 layer (FIG. 8), the photoresist is stripped andthe remaining oxide mask is removed by RIE.

After the POLY 1 layer 24 is etched, a second PSG layer (hereinafter“Second Oxide”) 28 is deposited (FIG. 9). The Second Oxide is patternedusing two different etch masks with different objectives.

First, a POLY1_POLY2_VIA etch (depicted at 30) provides for etch holesin the Second Oxide down to the POLY 1 layer 24. This etch provide amechanical and electrical connection between the POLY 1 layer and asubsequent POLY 2 layer. The POLY1_POLY2_VIA layer is lithographicallypatterned and etched by RIE (FIG. 10).

Second, an ANCHOR2 etch (depicted at 32) is provided to etch both theFirst and Second Oxide layers 18, 28 and POLY 1 layer 24 in one step(FIG. 11). For the ANCHOR2 etch, the Second Oxide layer islithographically patterned and etched by RIE in the same way as thePOLY1_POLY2_VIA etch. FIG. 11 shows the wafer cross section after bothPOLY1_POLY2_VIA and ANCHOR2 etches have been completed.

A second structural layer, POLY 2, 34 is then deposited at a thicknessof 1.5 μm, followed by a deposition of 200 nm of PSG. The wafer is thenannealed to dope the POLY 2 layer and reduce its residual film stresses.Next, the POLY 2 layer is lithographically patterned with a seventh maskand the PSG and POLY 2 layers are etched by RIE. The photoresist canthen be stripped and the masking oxide is removed (FIG. 13).

The final deposited layer in the MUMPs process is a 0.5 μm metal layer36 that provides for probing, bonding, electrical routing and highlyreflective mirror surfaces. The wafer is patterned lithographically withthe eighth mask and the metal is deposited and patterned using alift-off technique. The final, unreleased exemplary structure is shownin FIG. 14.

Lastly, the wafers undergo sacrificial release and test using knownmethods. FIG. 15 shows the device after the sacrificial oxides have beenreleased.

In preferred embodiments, the device of the present invention isfabricated by the MUMPs process in accordance with the steps describedabove. However, the device of the present invention does not employ thespecific masks shown in the general process of FIGS. 1–15, but ratheremploys masks specific to the structure of the present invention. Also,the steps described above for the MUMPs process may change as dictatedby the MEMS Technology Applications Center. The fabrication process isnot a part of the present invention and is only one of several processesthat can be used to make the present invention.

FIG. 16 is a diagrammatic side view of a microelectrical mechanicalstructure (MEMS) optical display system 50 illustrating an exemplaryoperating environment for an electrostatic actuator of the presentinvention. Display system 50 includes a light source 52 and reflector 54that direct illumination light to a condenser lens 58. A beam splitter60 receives the illumination light from condenser lens 58 and reflectsthe light toward a microlens array 62 having a two-dimensional array oflenslets 64 (only one dimension shown). Lenslets 64 of microlens array62 receive the illumination light and focus it through apertures 66 inan aperture plate 68 toward a microelectrical mechanical structure(MEMS) reflective modulator 70. Microlens array 62 could be formed as amolded array of plastic lenses or an array of holographic lenses, alsoreferred to as hololenses, or may be an assembled array of conventionalglass lenses.

MEMS reflective modulator 70 has a two-dimensional array ofmicroelectrical mechanical structure (MEMS) reflectors 72 that arepositioned opposite corresponding apertures 66 in aperture plate 68.Each MEMS reflector 72 corresponds to a picture element or pixel and isseparately controllable by a display controller 78 to selectivelyreflect illumination light back through an aperture 66 according to animage control signal, thereby to form a display image. For example, eachMEMS reflector 72 would direct light back through its aperture 66 for anamount of time in proportion to the brightness of the correspondingpixel for a given pixel period.

Light reflected by MEMS reflectors 72 through apertures 66 passesthrough lenslets 64 and beam splitter 60 to a rear surface 84 of atransmissive display screen 86 for viewing by an observer 88. In analternative implementation, a projecting lens array may be positionedbetween beam splitter 60 and transmissive display screen 86 to enlargeor reduce the optical field so that it provides a desired image size ontransmissive display screen 86. MEMS reflective modulator 70, apertureplate 68, and microlens array 62 may be considered a display engine 90that may be compactly and efficiently manufactured for a wide range ofapplications.

MEMS optical display system 50 has a number of advantages over commonlyavailable liquid crystal displays. For example, MEMS reflectivemodulator 70 does not require that the illumination light be polarized,in contrast to the typical operation of liquid crystal cells. Thiseliminates the expense and light attenuation that typically accompaniespolarization. Moreover, MEMS reflective modulator 70 can passunmodulated light with virtually no attenuation, whereas typical liquidcrystal cells significantly attenuate light. Similarly, MEMS reflectivemodulator 70 can provide much higher contrast ratios than liquid crystalcells because light is either losslessly reflected through apertures 66or completely blocked by aperture plate 68 to provide completemodulation of the light. Finally, MEMS reflective modulator 70 can bemanufactured by conventional CMOS circuit techniques without requiringthe complex processes typically required for liquid crystal displays.

In one implementation, for example, MEMS reflective modulator 70 couldinclude a 200×200 array of MEMS reflectors 72 for controlling lightpassing through a corresponding 200×200 array of apertures 66. In thisimplementation, for example, microlens array 62 could include 200×200lenslets 64 that each have a focal length of about 1 mm, and apertures66 may be positioned in a right, regular array with separations of about50 μm between them. MEMS reflective modulator 70 in such animplementation could have dimensions of 1 cm×1 cm. With lenslets 64 ofprojection microlens array 80 providing magnification of about 2.5,display screen 86 could have dimensions of about 2.5 cm×2.5 cm, or about1 inch×1 inch.

FIG. 17 is a diagrammatic side view of a microelectrical mechanicalstructure (MEMS) optical display system 150 showing one implementationof a polychromatic illumination source 152 and an associated reflector154. Components of MEMS optical display system 150 that are generallythe same as those of display system 50 are indicated by the samereference numerals.

Illumination source 152 includes multiple (e.g., three) color componentlight sources (e.g., lamps) 156R, 156G, and 156B that are positionedgenerally in a line and generate red, green, and blue light,respectively. A display controller 158 that separately controls MEMSreflectors 72 also activates color component light sources 156R, 156G,and 156B separately, generally known as field-sequential color. Duringtimes that it successively activates color component light sources 156R,156G, and 156B, display controller 158 applies control signals to MEMSreflectors 72 corresponding to red, green, and blue image components,thereby to form color component images in a field-sequential manner.

For example, color component images that are generated at a rate of 180Hz can provide an image frame rate of 60 Hz. In one exemplaryimplementation, a display of 200×200 multi-color pixels could employmicrolens arrays 62 with a 204×204 array of lenslets 64 to compensatefor different optical paths taken by different color components of lightforming the display gamut. Aperture plate 68 and MEMS reflectivemodulator 70 would include corresponding arrays of apertures 66 andreflectors 72, respectively. As an alternative implementation, it willbe appreciated that multiple successive colors of illumination could beobtained by a spinning color filter wheel and a white light source, asis known in the art.

FIGS. 18 and 19 are schematic side views of an electrostatic bimorphMEMS actuator 170 according to the present invention in respectiveactivated and relaxed states that can be used, for example, to controlMEMS reflector 72. FIG. 18 shows MEMS reflector 72 with an orientationbehind an associated aperture 66 generally perpendicular to a lightpropagation direction 172. In this activated, pixel ON state,illumination light directed through aperture 66 is reflected by MEMSreflector 72 back through aperture 66 to be included in a display image.FIG. 19 shows MEMS reflector 72 with an inclined or a tilted orientationrelative to light propagation direction 172. In this relaxed, pixel OFFstate, illumination light directed through aperture 66 is reflected byMEMS reflector 72 toward a solid region of aperture plate 68 to beblocked and excluded from a display image.

FIG. 20 is a plan view of MEMS actuator 170, and FIGS. 21 and 22 areside views of MEMS actuator 170 in its respective activated and relaxedstates.

MEMS actuator 170 includes a structural anchor 174 (FIGS. 21 and 22)that is secured to a substrate 176, which includes nitride layer 12. Inthe view of FIGS. 21 and 22, nitride layer 12 appears to be in segmentsdue to the schematic representation of electrical connections toelectrodes 190. FIG. 23 correctly illustrates electrical connections 191between electrodes 190 as being positioned on the nitride layer 12 andlaterally offset from flexible arm 178. Note that the nitride layer 12is formed continuously over substrate 176 and not as the segmentssuggested by FIGS. 21 and 22. One end 177 of a cantilevered flexible arm178 is secured to or formed integrally from anchor 174 and extends to afree or floating paddle end 180 that supports MEMS reflector 72 (e.g.,formed of gold). Flexible arm 178 includes a semiconductor (e.g.,polysilicon) arm base 181 and a residual stress layer 182 of a materialother than the semiconductor (e.g., polysilicon) of arm base 181.

Residual stress layer 182 is formed of a material (e.g., gold) that isselected to have a coefficient of expansion different from that of thesemiconductor (e.g., polysilicon) material of arm base 181. In theillustrated implementation, residual stress layer 182 is formed on topsurface of arm base 181. The differing thermal coefficients of expansionof arm base 181 and gold and residual stress layer 182 characterizeflexible arm 178 as a bimorph.

Optional flex scores 184 may extend into a top surface of arm base 181and extend across its length, either part way or completely (the formershown). Flex scores 184 are spaced apart from each other and aregenerally perpendicular to the length of flexible arm 178. In oneimplementation, residual stress layer 182 is formed between optionalflex scores 184 when they are present. Flex scores 184 may simply beformed as a product or consequence of the conformality of MEMSprocessing as depressions in the Poly2 layer made from producingstand-off dimples 202, which are described below.

MEMS actuator 170 includes one or more electrostatic activationelectrodes 190 that are formed in or on, but insulated from substrate176 at spaced-apart intervals along and underneath flexible arm 178.Activation electrodes 190 and flexible arm 178 are electricallyconnected to respective actuator controllers 192 and 194. An optionalmemory or lock electrode 196 is formed under floating paddle end 180 andelectrically connected to an optional memory controller 198.

In the activated, display pixel ON state illustrated in FIG. 21,complementary signals or electrical states are applied by actuatorcontrollers 192 and 194 to respective activation electrodes 190 andflexible arm 178 to impart electrostatic attraction between them. Theelectrostatic attraction between activation electrodes 190 and flexiblearm 178 functions to hold flexible arm 178 generally flat againstsubstrate 176. Separate activation of optional memory controller 198,connected to a memory electrodes 196 and 200, can then serve to holdflexible arm 178 generally flat against substrate 176 even after thecomplementary signals provided to activation electrodes 190 and flexiblearm 178 are relaxed.

Stand-off dimples 202 extending from flexible arm 178 toward substrate176 hold flexible arm 178 in spaced-apart relation to substrate 176 inthe activated, pixel ON state. Dimples 202 contact the electricallyinsulating (e.g., nitride layer) of substrate 176. A dimple 202 at theend of paddle end 180 also keeps reflector 72 flat (i.e., parallel tosubstrate 176) in the activated, pixel ON state, as well as keepingflexible arm 178 spaced apart from memory electrode 200. Stand-offdimples 202 may extend part way or completely across flexible arm 178.

In the relaxed, pixel OFF state illustrated in FIG. 22, complementarysignals or electrical states are not applied by actuator controllers 192and 194 to respective activation electrodes 190 and flexible arm 178, orthe complementary signals are insufficient to activate actuator 170.Likewise, optional memory controller 198 is not activated. Accordingly,residual stress between arm base 181 and residual stress layer 182serves to bend, tilt, or “curl” flexible arm 178 out of the plane of theunderlying substrate 176, as illustrated in FIG. 21.

In one implementation, reflector 72 in the relaxed, pixel OFF staterests at an orientation of about 12 degrees with respect to substrate176. In one implementation, a transition time of about 1 ms is needed toactivate or release actuator 170 (i.e., change between the relaxed,pixel OFF state and the activated, pixel ON state). It will beappreciated that this transition time can be changed, and substantiallyreduced.

FIG. 23 shows a schematic diagram of a 2×2 array 210 of actuators 170having a storage or memory capability to illustrate the operation ofactuators 170. The operation of array 210 is described with reference tothe following activation or control signals:

-   -   Vse=storage electrode voltage    -   Ry=mirror arm voltage for Row-y    -   Cx=actuation electrode voltage for Column-x

As one exemplary implementation, actuation of a single actuator 170 at alocation CxRy in array 210 (e.g., location C1R2) to activated, pixel ONstate is accomplished by applying a row activation voltage (e.g., +60volts) to a row electrode Ry (e.g., R2), which delivers the rowactivation voltage to the flexible arm 178 of each actuator in the row.A column activation voltage (e.g., −60 volts) is applied to a columnelectrode Cx (e.g., C1), which delivers the column activation voltage tothe activation electrodes 190 of each actuator 170 in the column. Theseexemplary row and column activation voltages establish at actuator 170at a location CxRy in array 210 (e.g., location C1R2) a voltagedifferential of 120 volts between flexible arm 178 and activationelectrodes 190. With a voltage differential of at least 114 volts neededfor actuation, the 120 volt differential is sufficient to activate theactuator 170.

The activation voltages need be applied only temporarily if a memory orstorage voltage is applied to memory electrode 200 to establish adifferential relative to the row activation voltage delivered toflexible arm 178. In particular, the activation voltages need be appliedonly long enough (e.g., 1 ms in one implementation) for flexible arm 178to be deflected to and held by memory electrode 200.

With row and column electrodes other than row electrode Ry and columnelectrode Cx held at a median potential (e.g., 0 volts in oneimplementation), application of the row activation voltage to activationrow electrode Ry and column activation voltage to column electrode Cxwill function to activate only actuator 170 at location CxRy. WithRy=+60 volts and Cx=−60 volts, for example, other actuators 170 in rowRy and column Cx will each receive a voltage differential of only 60volts, which is insufficient for actuation. Moreover, with storage ormemory electrode 200 energized, all actuators 170 not specifically beingactivated will retain their previous states. For example, storage ormemory electrode 200 may be energized with a voltage of +60 volts, forexample. Such a memory or storage potential establishes between allstorage electrodes 200 and actuators 170 that are not specifically beingactivated or addressed a differential of at least 25 volts that issufficient to hold them in the pixel ON state.

Actuators 170 may be considered to have an actuation (or activation)state in which reflector 72 is moved into a flat (i.e., parallel tosubstrate 176) position for the activated, pixel ON state, a releasestate in which reflector 72 is released from the flat (i.e., parallel tosubstrate 176) position and curls out-of-plane for the relaxed, pixelOFF state, and a storage state in which in which reflector 72 is held inthe flat (i.e., parallel to substrate 176) position after actuation. Theactuation state and the storage state may be represented by thefollowing equations:A _(xy) =|R _(y) −C _(x)|=actuation potential difference for mirror RxCyH _(xy) =|R _(y) −V _(se)|=hold potential difference via storageelectrode for mirrors on R _(y)Actuation (from the released state)

The mirror will transition to the actuated (down, parallel to thesubstrate) state only if

-   -   A_(xy)>114 volts.        Release (from the actuated state)

The mirror will transition to the released (up) state only if

-   -   A_(xy)<53 volts and H_(xy)<25 volts        Storage

The mirror will retain its current state only if

-   -   A_(xy)>53 volts or H_(xy)>25 volts if state=actuated    -   A_(xy)<114 volts if state=released

FIG. 24 is a fragmentary schematic diagram of a 50×50 array 230 ofactuators 170 having a storage or memory capability. Array 230 employs50 row electrodes 232 that are coupled to corresponding row drivers 234,50 column electrodes 236 that are coupled to corresponding columndrivers 238, and a common storage electrode 240 for all actuators 170connected to a storage driver 242. It will be appreciated that rowdrivers 234 and column drivers 238 may include an individual driver foreach of respective electrodes 232 and 236, or may include a lessernumber of drivers that are multiplexed among the electrodes. Drivers234, 238, and 242 and other electronics can be formed on substrate 176or as separate devices, depending complexity, packaging and economics. Adisplay processor 244 receives display signals from a display input 246and provides corresponding control signals to drivers 234, 238, and 242.

In this implementation, row drivers 234 switch between 0 and +60 volts,and column drivers 238 switch between 0, +60 and −60 volts. Storageelectrode driver 242 switches between 0 and +60 volts. If the actuators170 are sequentially addressed, a period of about 50×50×1 ms, or 2.5seconds, will be needed to address all actuators 170 in array 230. Itwill be appreciated that only those actuators 170 with a reflector 72requiring a state change need be addressed. Accordingly, less time wouldbe required if fewer than all actuators 170 were to be addressed. If 50row drivers 234 and 50 column drivers 238 are employed, whole rows orcolumns can be addressed simultaneously, and the actuators 170 in therows or columns activated in parallel, thereby decreasing the arrayaddressing period to about 50 ms.

FIG. 25 is a flow diagram of a row-sequential addressing method 250described with reference to the exemplary implementation of actuator 170and array 230 described above. It will be appreciated thatrow-sequential addressing method 250 could be readily adapted to otherimplementations of actuators 170 and arrays 230.

Step 252 indicates a “Clear All” step in which the reflectors 72 of allactuators 170 in array 250, for example, are returned to a relaxed, nopixel off state. The Clear All of step 252 is optional and may berepresented by the following drive voltages:Vse=Ry=Cx=0 volts (for all x and y).

Step 254 indicates an “Arm Storage” step in which the storage electrodes240 for all actuators 170 are activated or energized by storage driver242. The Arm Storage of step 254 may be represented by the followingdrive voltage:Vse=+60 volts.

Step 254 applies +60 volts to storage electrodes 240 so any activatedactuator 170 is held in its activated, pixel ON state until released.This step does not affect actuators in the relaxed, pixel OFF becausethe activation of storage electrodes 240 is insufficient to activate anactuator 170 in the relaxed state.

Step 256 indicates a “Begin” step in which a counter of rows or columns(e.g., rows) is initialized at a value “1.” Step 258 sets the rowcounter “i” to a first row of actuators 170 in array 230. The Begin ofstep 256 may be represented as:Set “i”=1.

Step 258 indicates a “Row-i Set” step in which the storage electrodes240 for all actuators 170 are activated or energized by storage driver242. The Row-i Set of step 258 may be represented by the following drivevoltages:Set Ry=i=+60 volts,Cx=data states for Ry=iC1, Ry=iC2, . . . , Ry=iC50.To activate actuators 170 within Row-i (Ry=i=+60 volts), set Cx=−60volts, giving an activation differential:(Ax,y=i=|60−(−60)|=120 volts=>mirror RxCy is actuated).To release actuators 170 within Row-I, (Ry=i=+60 volts), set Cx=+60volts, giving a release differential:(Ax,y=j=|60−60|=0 volts, Hx,y=j=|60−60|=0 volts (no hold)For mirrors other than in Row-i (Ry≠i=0 volts)

Ax, y≠ i = |0 − Rx| = (0 or 60) volts, insufficient for actuation. Hx,y≠ i = |0 − 60| = 60 volts sufficient for holding statesIt will be noted that there is no hold electrode function within Row-iin this step, since Hx,y=i=0 volts. Accordingly, step 258 sets (orreleases previously actuated and held) the states of actuators 170 inRow-i to their required new positions and holds them in these stateswhile voltages in step 258 are held.

Step 260 indicates a “Hold All” step in which all row and columnelectrodes are set to a neutral potential (e.g., 0 volts) and thestorage electrodes 240 for all actuators 170 are activated or energizedby storage driver 242 sufficient for holding the current states. TheHold All of step 260 may be represented by the following drive voltages:All Ry=0 volts and all Cx=0 voltsHxy=60 volts

Step 262 indicates an “Increment Row” step in which counter “i” isincremented by a count of one. Step 262 returns to step 258 repeatedlyuntil all rows are addressed (e.g., until count “i”=50). Step 262 theproceeds to step 264.

Step 264 indicates a “Repeat” step that returns to step 256.

In the exemplary implementation described above, an activation potentialof greater than 114 volts between flexible arm 178 and activationelectrodes 190 establishes sufficient electrostatic force to pull curledflexible arm 178 in its relaxed state toward substrate 176. As a result,dimples 202 are pulled against substrate 176 and reflector 72 lies flat,nominally at 0 degrees, in an activated, pixel ON state.

FIG. 26 is a graph 280 illustrating hysteresis characteristics of anactuator 170 with respect to applied voltage differentials. As shown inFIG. 26, actuator 170 exhibits hysteresis effects such that afteractivation at about 114 volts, the applied voltage differential needs tobe reduced below 53 volts for actuator 170 to be released.

A bottom horizontal segment 282 illustrates that the voltagedifferential between activation electrodes 190 and flexible arm 178needs to be increased above about 114 volts to move flexible arm 178from a relaxed upward state to an activated downward state, thetransition being illustrated by vertical segment 284. A top horizontalsegment 286 illustrates that the voltage differential then needs to bedecreased below about 53 volts to move flexible arm 178 from anactivated downward state to a relaxed upward state, the transition beingillustrated by vertical segment 288.

A storage activated segment 290 illustrates activation of storageelectrodes 240 and shows that flexible arm 178 remains in the activateddownward state even as the voltage differential between activationelectrodes 190 and flexible arm 178 is reduced to below 53 volts. In theillustrated implementation, activation of storage electrodes 240includes a voltage difference of greater than 25 volts (e.g., 60 volts)between storage electrode 240 and flexible arm 178. In contrast, astorage not-activated segment 292 illustrates absence of activation ofstorage electrodes 240 (e.g., 0 volt) and shows flexible arm 178returning to its relaxed upward state when the voltage differentialbetween activation electrodes 190 and flexible arm 178 is reduced tobelow 53 volts.

It will be appreciated that the hysteresis of actuator 170 allows it tobe operated and held in its activated downward state without use ofstorage electrodes 240. Such an operation requires closer tolerances asto the row and column drive voltages and can be more sensitive toprocess or manufacturing variations that may cause operation failure.Nevertheless, actuator 170 can be operated without storage electrodes240 to provide a simpler layout and to eliminate the requirement forstorage drivers.

With reference to the exemplary MUMPS manufacturing process, arm base181 may be formed as a singly clamped cantilever of 1.5 μm thickpolysilicon patterned on the Poly2 layer. Activation electrode 190 maybe formed from the Poly0 layer. Dimples 202 may be formed in the Poly2layer with Anchor1 (i.e., a hole in the 2 μm Oxide1 layer) to provide astand-off of 2 μm, thereby to prevent the major bottom surface offlexible arm 178 from electrically contacting activation electrode 190when actuated. Residual stress layer 182 may be formed as a 0.5 μm thicklayer of gold. Reflector 72 may also be coated with gold to increaseoptical reflectance.

Paddle end 180 of flexible arm 178 may be formed to resist the curlingfrom residual stress characteristic of the rest of arm 178. FIG. 27 is aschematic sectional side view of one implementation paddle end 180formed as a relatively thick composite structure 300 of, for example, a1.5 μm thick layer 302 of Poly2 (i.e., the material of arm base 181), aswell as a 2 μm thick layer 304 of Poly1 (which includes dimples 202) anda 0.75 μm thick layer 306 of trapped Oxide2 between layers 302 and 304.

Parts of the description of the preferred embodiment refer to steps ofthe MUMPs fabrication process described above. However, as stated, MUMPsis a general fabrication process that accommodates a wide range of MEMSdevice designs. Consequently, a fabrication process that is specificallydesigned for the present invention will likely include different steps,additional steps, different dimensions and thickness, and differentmaterials. Such specific fabrication processes are within the ken ofpersons skilled in the art of photolithographic processes and are not apart of the present invention.

In view of the many possible embodiments to which the principles of ourinvention may be applied, it should be recognized that the detailedembodiments are illustrative only and should not be taken as limitingthe scope of our invention. Rather, I claim as my invention all suchembodiments as may come within the scope and spirit of the followingclaims and equivalents thereto.

1. A microelectrical mechanical actuator, comprising: a planar substratewith an anchor secured thereto; cantilevered flexible bimorph armsecured at one end to the anchor and extending along and over thesubstrate, the bimorph arm including a distal end opposite the endsecured to the anchor; a substrate electrode secured to and insulatedfrom the substrate and positioned under and in alignment with thebimorph arm; activation electrical couplings to receive an electricalpotential difference between the bimorph arm and the substrate electrodeto impart electrostatic attraction between the bimorph arm and thesubstrate electrode, thereby to activate the actuator; a memoryelectrode secured to and insulated from the substrate and positionedunder and in alignment with the distal end of the bimorph arm; and amemory electrical coupling to receive an electrical potential at thememory electrode relative to the bimorph arm to impart electrostaticattraction between the bimorph arm and the memory electrode separatelyfrom the potential difference applied between the bimorph arm and thesubstrate electrode.
 2. The actuator of claim 1 in which the bimorph armincludes a layer of semiconductor material and a layer of anothermaterial having a coefficient of expansion different from that of thesemiconductor material to impart a differential residual stress in thebimorph arm.
 3. The actuator of claim 2 in which the residual stressinduces an inclination for the bimorph arm to curl out-of-plane awayfrom the substrate when the actuator is relaxed.
 4. The actuator ofclaim 1 further including multiple substrate electrodes secured to thesubstrate and positioned under and in alignment with the bimorph arm. 5.The actuator of claim 1 in which the bimorph arm further comprises areflective surface positioned at an end of the bimorph arm opposite theend secured to the anchor.
 6. The actuator of claim 1 further comprisinga source of an electrical potential connected to the memory electricalcoupling to provide the potential to the memory electrical couplingrelative to the bimorph arm.
 7. The actuator of claim 1 furthercomprising one or more stand off dimples extending from the bimorph armtoward the substrate to hold the bimorph arm in spaced-apart relation tothe substrate when the actuator is activated.
 8. The actuator of claim 1further comprising a source of an electrical potential connected to theactivation electrical couplings to provide the potential difference tothe activation electrical couplings to activate the actuator.
 9. In amicroelectrical mechanical actuator formed on a planar semiconductorsubstrate, the improvement comprising: a cantilevered flexible bimorpharm that is secured to the planar substrate and selectively positionablein an relaxed state and in an electrostatically activated state, thebimorph arm being generally parallel to the planar substrate in theelectrostatically activated state and extending out-of-plane from theplanar substrate in the relaxed state, the bimorph arm being secured atone end to an anchor that is secured to the substrate, the bimorph armincluding a length from the end secured to the anchor to a distal end; amemory electrode secured to the substrate and positioned under and inalignment with the distal end of the bimorph arm; and a memoryelectrical coupling to receive an electrical potential at the memoryelectrode relative to the bimorph arm to impart electrostatic attractionbetween the bimorph arm and the memory electrode to maintain the bimorpharm in the activated state.
 10. The actuator of claim 9 in which thebimorph arm includes a layer of semiconductor material and a layer ofanother material having a coefficient of expansion different from thatof the semiconductor material to impart a residual stress in the bimorpharm.
 11. The actuator of claim 10 in which the residual stress inducesan inclination for the bimorph arm to curl out-of-plane away from thesubstrate when the actuator is relaxed.
 12. The actuator of claim 9 inwhich the bimorph arm is secured at one end to an anchor that is securedto the substrate, the bimorph arm further comprising a reflectivesurface positioned at an end opposite the end secured to the anchor. 13.The actuator of claim 9 further comprising: a substrate electrodesecured to the substrate and positioned under and in alignment with thebimorph arm; and activation electrical couplings to receive anelectrical potential difference between the bimorph arm and thesubstrate electrode to impart electrostatic attraction between thebimorph arm and the substrate electrode, thereby to activate theactuator.
 14. The actuator of claim 13 further including multiplesubstrate electrodes secured to the substrate and positioned under andin alignment with the bimorph arm.
 15. The actuator of claim 9 furthercomprising one or more stand-off dimples extending from the bimorph armtoward the substrate to hold the bimorph arm in spaced-apart relation tothe substrate when the actuator is activated.
 16. A microelectricalmechanical actuator array, comprising: a plurality of electrostaticbimorph microelectrical mechanical actuators that each have cantileveredflexible bimorph arm that is secured to a planar substrate andselectively positionable in an activated state and in a relaxed state,the bimorph arm being generally parallel to the planar substrate in theactivated state and extending out-of-plane from the planar substrate inthe relaxed state, each bimorph arm being secured at one end to thesubstrate and including a length from the end secured to the substrateto a distal end; a substrate electrode secured to the substrate andpositioned under and in alignment with the bimorph arm of each actuator;activation electrical couplings to apply an electrical potentialdifference between the bimorph arm and the substrate electrode of eachactuator, the electrical potential difference activating each actuatorwith electrostatic attraction between the bimorph arm and the substrateelectrode; a memory electrode secured to the substrate and positionedunder and in alignment with the distal end of the bimorph arm; and amemory electrical coupling to receive an electrical potential at thememory electrode relative to the bimorph arm to impart electrostaticattraction between the bimorph arm and the memory electrode to maintainthe bimorph arm in the activated state.
 17. The actuator array of claim16 in which the memory electrical couplings of all the actuators areelectrically connected together.
 18. The actuator array of claim 16 inwhich each actuator is in a first aligned set of actuators and a secondaligned set of actuators, the first and second aligned sets of actuatorsextending in transverse first and second directions, respectively, andthe array including plural first and second aligned sets of actuators.19. The array of claim 18 in which the bimorph arms of the actuators ineach first aligned set of actuators are electrically connected togetherand the substrate electrodes of the actuators in each second aligned setof actuators are electrically connected together.
 20. The actuator arrayof claim 18 in which the first and second aligned sets of actuators aregenerally orthogonal to each other and form respective rows and columnsof actuators.
 21. A microelectrical mechanical actuator, comprising: aplanar substrate with an anchor secured thereto; cantilevered flexiblebimorph arm secured at one end to the anchor and extending along andover the substrate; a substrate electrode secured to and insulated fromthe substrate and positioned under and in alignment with the bimorpharm; activation electrical couplings to receive an electrical potentialdifference between the bimorph arm and the substrate electrode to impartelectrostatic attraction between the bimorph arm and the substrateelectrode, thereby to activate the actuator; plural substrate electrodessecured to the substrate and positioned under and in alignment with thebimorph arm; and plural stand-off dimples extending between the bimorpharm and the substrate to hold the bimorph arm in spaced-apart relationto the substrate when the actuator is activated, the plural stand-offdimples being interdigitated with the plural substrate electrodes whenthe actuator is activated.
 22. In a microelectrical mechanical actuatorformed on a planar semiconductor substrate, the improvement comprising:a cantilevered flexible bimorph arm that is secured to the planarsubstrate and selectively positionable in a relaxed state and in anelectrostatically activated state, the bimorph arm being generallyparallel to the planar substrate in the electrostatically activatedstate and extending out-of-plane from the planar substrate in therelaxed state; plural spaced-apart substrate electrodes secured to thesubstrate and positioned under and in alignment with the bimorph arm;and plural stand-off dimples extending between the bimorph arm and thesubstrate to hold the bimorph arm in spaced-apart relation to thesubstrate when the actuator is activated, the plural stand-off dimplesbeing interdigitated with the plural substrate electrodes when theactuator is activated.